The present invention relates to a laminated rubber bearing. More particularly, the invention relates to a peripheral-binding type laminated rubber bearing which is capable of supporting a variety of structures including buildings, bridges, tanks, or the like, or precision apparatuses including computers, medical-treatment apparatuses, security apparatuses, high-precision machine tools, analytical apparatuses, or the like, or artifacts, in the state of stably mounting those objects thereon in order to minimize acceleration incurred from earthquake, mechanical vibration, or traffic vibration, for example.
For instance, conventionally, there is such a laminated rubber bearing used for decreasing acceleration transmitted to those structures, precision apparatuses, or artifacts cited above by way of functioning as a vibration-proof supporter. This conventional laminated rubber bearing has a structure composed of alternately laminated rigid plates made of steel and soft rubber-like elastic plates having negligible effect of compressive permanent strain. This conventional laminated rubber structure is disposed between upper and lower structures to swingably support the upper structure in the horizontal direction in order to decrease acceleration incurred from earthquake and protect the upper structure from destructive force generated by earthquake.
As is apparent from the above description, damping performance and stability against vertical load (hereinafter referred to as "buckling-proof" performance) are extremely important factors in any of those laminated rubber bearings used for protection against destructive force generated by earthquake. Examples of damping performance and buckling-proof performance of conventional laminated rubber bearings are described below.
If a laminated rubber bearing were inappropriate in the damping performance, then, it will not be able to exert enough capability to fully absorb vibration energy in actual service. This means that it will take a long while before horizontal swing movement generated by vibration can fully be terminated, and thus, this bearing is inadequate for practical service. Therefore, in order to promote damping performance, in addition to such a laminated rubber bearing, external dampers are conjunctionally used. In addition, there is such a newly developed laminated rubber bearing using rubber-like plates made from high-damping rubber. Details of such a newly developed laminated rubber bearing incorporating practical means for promoting damping performance are described below.
A conventional laminated rubber bearing disclosed in the Japanese Patent Publication No. 60-47417 has such a structure containing space in the structural contour of rubber-like elastic plate layers, where the space is filled with viscoelastic substance. In consequence, vibration-damping performance is promoted by means of deforming energy of the internally accommodated viscoelastic body.
Another conventional laminated rubber bearing disclosed in the Japanese Laid-Open Patent Publication No. 64-58370 has such a peripheral-binding type structure capable of exerting high rigidity in the vertical direction and high potential for supporting load in the state of exerting substantial deforming capability in the horizontal direction by filling a through-hole with a columnar viscoelastic body to hold this body with a peripheral binding body. In addition, the peripheral binding body and/or the viscoelastic body are respectively capable of absorbing vibration energy mainly generated by frictional attenuation.
Another conventional laminated rubber bearing disclosed in the Japanese Laid-Open Utility Model No. 2-54933 has such a structure described below. An elastic supporter is formed by alternately laminating a plurality of rigid plates made of steel and a plurality of elastic plates made of soft rubber. Viscous substance is accommodated in a through-hole extending in the axial direction of the elastic supporter. The viscous substance and the elastic substance are nipped by an upper plate an d a lower plate. The viscous substance integrated with these components is interposed between an objective structure and a supporter used for supporting the objective structure. Those steel plates respectively have inner circumferential surfaces extending to the interior of the through-hole. This structure increases tangential area between those steel plates and the viscous substance, thus resulting in the intensified frictional force and promoted damping capability.
Next, buckling-proof performance is described below. If buckling-proof performance of a laminated rubber bearing were inappropriate, then, the bearing will be subject to buckling caused by vertical load after being sheared by destructive force of earthquake. Critical buckling point of this bearing is determined based on the relationship between the displacement of the sheared supporter and horizontal load when shearing occurs. When displacement caused by shearing exceeds the critical buckling point, the effect of displacement caused by shearing is quite noticeable. Therefore, such a conventional laminated rubber bearing apt to be subject to buckling may incur grave damage not only to the bearing itself but also to the structure mounted thereon, and therefore, such a conventional laminated rubber bearing cannot be offered for use in practical fields. In the case of such a conventional peripheral-binding type laminated rubber bearing comprising alternately laminated rigid plates and rubber-like elastic members and a through-hole formed in the direction of height of the laminated body, buckling-proof performance is gravely affected by secondary shape factor and inner diameter of the through-hole permeating the rigid plates. Concretely, the conventional laminated rubber bearing is subject to buckling when a negligible amount of shearing displacement takes place in such a case in presence of a small value of secondary shape factor or in such a case in presence of a large proportion of the inner diameter of the through-hole against outer diameter of the rigid plates.
Referring to the relationship between damping performance and buckling-proof performance of the peripheral-binding type conventional laminated rubber bearing having a through-hole of the rigid plates filled with a viscoelastic body and plastic substance, if inner diameter of the through-hole were expanded in order to accommodate a greater amount of viscoelastic or plastic substance merely for promoting damping performance, then, it will conversely degrade buckling-proof performance. In consequence, such a conventional laminated rubber bearing cannot be offered for practical service. On the other hand, if the inner diameter of the through-hole were contracted for promoting buckling-proof performance, then, it will result in the contracted cubage or volume of the viscoelastic body or the plastic substance. In consequence, this conventional laminated rubber bearing cannot secure the needed damping performance. In this way, magnitude of the inner diameter of the through-hole inversely affects the damping performance and the buckling performance in contradiction to each other.
For example, another conventional laminated rubber bearing disclosed in the Japanese Laid-Open Patent Publication No. 3-163231 has such a structure described below. By effect of filling a through-hole with a viscoelastic body having cubage greater than that of the through-hole of the laminated body, the viscoelastic body having free surface being bound in the inner surface of the through-hole swells towards the rubber-like elastic plates by such an amount corresponding to surplus cubage. Expansion of the viscoelastic body between rigid plates enables viscoelastic substance to fully adhere to the inner peripheral surface of the through-hole. This in turn results in the satisfactory mechanical coupling effect between the viscoelastic substance and the laminated body, thus promoting buckling-proof performance while properly maintaining high damping performance.
The above-cited Japanese Laid-Open Patent Publication No. 64-58370 also discloses structure of another laminated rubber bearing, where a viscoelastic body filled in a through-hole comprises a laminated viscoelastic body composed of alternately laminated viscoelastic bodies and rigid plates. This structure promotes both the damping performance and the buckling-proof performance.
On the other hand, those conventional laminated rubber bearings cited above respectively have technical problems to solve, details of which are described below.
The above-cited Japanese Patent Publication No. 60-47417 discloses such a laminated rubber bearing incorporating a viscoelastic body inserted between a plurality of steel plates in order to generate damping performance. However, according to the structure disclosed therein, unlike the one disclosed in the above-cited Japanese Laid-Open Patent Publication No. 3-163231, this structure cannot fill the viscoelastic body between each steel plate by way of exceeding cubage, and therefore, clearance is generated in the inserted viscoelastic body. This in turn lowers capability of the laminated rubber bearing to support vertical load, buckling-proof performance, and absorption of energy generated by shearing deformation. Furthermore, the Japanese Laid-Open Patent Publication No. 3-163231 discloses a technique to fill viscous substance between respective steel plates in order to generate damping performance. However, introduction of the viscous substance does not significantly contribute to the damping constant, and thus, this conventional laminated rubber bearing cannot generate substantial damping effect.
Referring to the conventional laminated rubber bearing disclosed in the above-cited Japanese Laid-Open Patent Publication No. 64-58370, as mentioned earlier, dimension of inner diameter of a through-hole formed in the bearing inversely affects the damping performance and the buckling-proof performance in contradiction to each other, and thus, there is a certain limit in the improvement of the damping performance while properly maintaining buckling-proof performance effective for practical service.
On the other hand, referring to the conventional laminated rubber bearing disclosed in the above-cited Japanese Laid-Open Utility Model No. 2-54933, it accommodates viscous substance in a through-hole extending in the axial direction of an elastic supporter, and in addition, both the viscous substance and the elastic supporter are merely nipped by an upper plate and a lower plate without filling a viscoelastic body by way of exceeding cubage of the through-hole formed therein. Therefore, like the one disclosed in the Japanese. Patent Publication No. 60-47417 described earlier, structure of this conventional laminated rubber bearing causes buckling-proof performance to be degraded. Furthermore, since this conventional laminated rubber bearing merely uses viscous substance, it is quite apparent that this system can hardly be offered for practical service by way of fully enhancing damping performance thereof.
Finally, referring to the conventional laminated rubber bearing disclosed in the above-cited Japanese Laid-Open Patent Publication No. 3-163231, in the event that such an extremely disastrous earthquake ever occurs like the "Kanto Earthquake" which occurred on Sep. 1, 1923, by directly striking Tokyo and surrounding Kanto area of Japan and generated approximately 25 through 30 cm of shearing deformation and 150% through 180% of shearing strain, due to flow of viscoelastic substance, cavity can easily be generated inside of the viscoelastic body, and therefore, the above conventional laminated rubber bearing cannot substantially improve buckling-proof performance suited for practical service. At the same time, structure of this conventional laminated rubber bearing lowers proportion in the shearing and deformation of the viscoelastic body, thus lowering both the energy-absorptive capability and the damping performance. Furthermore, while making use of the laminated type viscoelastic body disclosed in the above-cited Japanese Laid-Open Patent Publication No. 3-163231, height of all the rigid plates built in this laminated type viscoelastic body must fully be arranged to be flush with the rigid plates of the laminated body. If they were not fully flush with each other, then, the viscoelastic body cannot fully swell itself towards the rubber-like elastic plates to result in the difficulty to achieve satisfactory mechanical coupling condition to eventually lower buckling-proof performance.